Normal magnetic material has a magnetic hysteresis loop characteristic and the relation of magnetizing force and magnetic flux density is nonlinear. As a result, when magnetic flux of a voice coil passes through a center pole and a plate near an air gap, the magnetic reaction in the voice coil results in a distorted output signal from the speaker.
A conventional speaker of the type contemplated by the present invention is illustrated in FIG. 1. The unit shown in FIG. 1 has a pole yolk 12 with an inverted T-cross section which has around its bottom an annular magnet 14 that is magnetized in the direction of its thickness. A top plate 18 rests on magnet 14 to form a magnetic gap with the peripheral wall of center pole 16 of pole yolk 12. A voice coil 28 wound around a bobbin 30 is inserted into the magnetic gap. Bobbin 30 is supported on a frame 26 by a damper 24 that permits vibratory movement of bobbin 30. A diaphragm 22 with a dust cover 20 is connected to bobbin 30. The outer periphery of diaphragm 22 is supported by an edge portion 32 that is fixed to frame 26 by a gasket 34. In order to make effective use of the magnetic flux in the gap, voice coil 28 is wound around bobbin 30 a length in the longitudinal direction of the coil which is greater than the magnetic gap link. Center pole 16 of pole yolk 12 is fitted with a cap 36 provided to reduce the distortion that is introduced into the coil current by the inductance of voice coil 28. The operation of the speaker shown in FIG. 1 is as follows. When a current flows through voice coil 28, either an upward or downward driving force acts on coil 28 depending upon the direction of the current flow. Since voice coil 28 is mobile, the driving force acting on voice coil 28 is transmitted to diaphragm 22 through bobbin 30. As a result, diaphragm 22 pushes the air in front of it, creating sound waves.
Various methods of eliminating the distortion described above have been attempted, such as covering a center pole with a copper cap as described in FIG. 1, and putting a copper ring around the center pole. The copper ring requires a higher level of manufacturing technique and tends to disturb the magnetic flux distribution in the speaker's air gap thus interfering with the output signal. The application of a copper cap covering the center pole of a speaker has been more successful but also has several drawbacks.
The pole/cap configuration and processes used in the prior art for coupling the cap have not resulted in an easily manufactured assembly which performs well, particularly in high volume production environments. FIG. 2 is directed to one prior art method for coupling the cap described briefly in FIG. 1, to a speaker center pole to reduce the distortion introduced into the speakers voice coil due to inductance are shown. FIGS. 2a and 2b demonstrate different methods of coupling a distortion reduction cap to center pole 16 of pole yolk 12. In both methods the cap must be formed on cap die press 40. FIG. 2a shows the prior art method of coupling distortion reduction cap 46 with adhesives at 42. Once cap 46 is formed by cap die press 40, applicator 48 applies an adhesive, such as glue, within cap 46 which in turn is coupled onto center pole 16 of pole yolk 12 to form partial speaker assembly 50. Use of an adhesive requires additional manufacturing steps to provide the correct amount of adhesive to ensure that center pole 16 receives an even coating to securely couple cap 46. Distortion reduction cap 46 must also be properly coupled to center pole 16 to ensure no air gaps are formed between cap 46 and center pole 16. Such gaps reduce the effectiveness of cap 46.
In order for distortion reduction cap 46 to provide maximum reduction in distortion there must not only be no air gaps between cap 46 and center pole 16 but cap 46 must also remain absolutely stationary. Adhesives used in the prior art are problematic because when subjected to varying temperatures and environmental conditions they can loosen and allow cap 46 to vibrate and move again creating air gaps which effect the distortion reduction performance of cap 46. FIG. 2b, at 44, shows an alternate method found in the prior art for coupling a distortion reduction cap to a speaker center pole. The process begins with cap 52 being formed on cap die press 40. Distortion reduction cap 52 is shown in cross-section and is identical to cap 46 with the exception of an annular inwardly extending lip 54. Instead of using an adhesive to maintain the coupling between cap 52 and center pole 16 of yolk 12, the coupling is instead maintained by forcing the lower portion of cap 52 into axial inwardly formed groove 56 within center pole 16. The portion of cap 52 forced within groove 56 becomes annular inwardly extending lip 54. This method is inadequate as it does not prevent cap 52 from rotating upon center pole 16 once retained in place, and may result in air gaps between cap 52 and center pole 16. Vibration from sound waves and due to imperfect road conditions, if the speakers are used in automobiles, may cause such rotation which allows air gaps between cap 52 and center pole 16 to again reduce the effectiveness of distortion reduction cap 52.
Although not previously discussed, larger conventional type speakers of the type contemplated by the present invention use a center pole which includes a through hole through the center axis of the center pole. Another method for reducing distortion is to keep dust and debris from entering this chamber and affecting the diaphragm. A dust screen is typically attached to the pole yolk with an adhesive. Adhesives used in the prior art are problematic because when subjected to varying temperatures and environmental conditions they can loosen providing gaps around the edges of the screen or disengaging the screen altogether which will allow dust and other contaminants into the speaker affecting the diaphragm and resulting in increased distortion.
Therefore, in light of the foregoing deficiencies in the prior art, the applicant's invention is herein presented.